This invention relates to an improved electronic package and a curable material useful as a thermal material therein.
Electronic components such as integrated circuit chips can generate sufficient heat so that a heat dissipation arrangement must be provided. A common expedient for this purpose is to transfer heat from the component using a thermally conductive member, for example an integrated heat spreader and/or a heat sink, thermally connected to the circuit board or component. A thermal interface material (TIM) is used between the component or circuit board and the thermally conductive member to establish thermal contact and lower the thermal resistance. The TIM technologies used for electronic packages encompass several classes of materials such as phase change materials, epoxies, greases, and gels.
Due to the increasing performance demands for electronic components such as microprocessors, improving heat dissipation is one of the central issues. The recent trend in microprocessor architecture has been to increase the number of transistors (higher power), shrink processor size (smaller die), and increase clock speeds (high frequency) in order to meet the market demand for high performance microprocessors. These have resulted in the escalation of both the raw power as well as the power density (hot spots) at the silicon die level, which increase the demand for effective means of heat dissipation.
High performance, high power processors require the use of integrated heat spreaders. The well-known thermal greases, epoxies and phase change TIM materials that are currently available in the market do not meet the performance requirement for packages comprising an integrated heat spreader. In response, highly conductive, low modulus, crosslinked gel TIMs were developed.
U.S. Pat. No. 6,238,596 discloses a method of improving the thermal conductivity of gel TIM polymer systems by incorporating carbon microfibers, with other fillers, in the thermal interface material. Other solutions to the demand for increasingly effective heat dissipation have also been proposed. In U.S. Pat. No. 6,218,730, by means of mechanical standoffs, the TIM gap is reduced to provide a shorter heat transfer path and thereby a reduction in the thermal resistance of the TIM. Improvements in packaging design include those disclosed in Assignee""s U.S. Pat. No. 6,188,576, which account for varying amounts of heat generated by separate chips within a package. Imparting a consistent TIM thickness and thereby allowing the uniform transfer of heat are named advantages of the techniques for the application of TIMs by screen printing the TIM composition upon a substrate to form a layer followed by curing the layer, disclosed in U.S. Pat. Nos. 6,020,424 and 6,210,520.
A method of making a heat dissipation arrangement involving the formation of a gel pad on the inner surface of a heat spreader to cover exposed faces of chips on a circuit board is described in U.S. Pat. No. 6,162,663. Properties of the gel pad are specified to dissipate heat while at the same time physically protecting the chip from mechanical stresses or avoiding the transmission of such stresses to the bare silicon chips. A cured gel TIM to form the pad is specified to have a cohesive strength greater than its adhesive strength, a compressive modulus of less than 1.38 MPa, and a thermal conductivity of greater than 1.0 W/mxc2x0 C.
Gel TIMs typically comprise a crosslinkable silicone polymer, such as a vinyl-terminated silicone oil, a crosslinker, such as a silane hydride crosslinker, and a thermally conductive filler. Before cure, these materials have properties similar to greases. They have high bulk thermal conductivities and low surface energies, and they conform well to surface irregularities upon dispense and assembly, which contributes to thermal contact resistance minimization. After cure, gel TIMs are crosslinked, filled polymers, and the crosslinking reaction provides cohesive strength to circumvent the pump-out issues exhibited by greases during temperature cycling. Their modulus (E) is low enough (on the order of mega-pascal, MPa, range compared to giga-pascal, GPa, range observed for epoxies) that the material can still dissipate internal stresses and prevent interfacial delamination. Thus, the low modulus properties of these filled gels are attractive from a material integration standpoint.
However, it is often found that maintaining low thermal interface resistance in electronic packages employing gel TIMs currently used in the industry, is difficult. This is especially true for organic flip-chip packages, which introduce significant thermal-mechanical stress on the thermal interface material during reliability stress testing from the relative flexing of the die and the heat spreader with changes in temperature due to the differences in their coefficients of thermal expansion. One of the main technical challenges for gel TIM formulation is optimizing the mechanical properties such that the cured gel dissipates the thermal-mechanical stresses that arise due to the mismatch of thermal expansion coefficients of the chip and heat spreader, to thereby avoid delamination of the gel TIM. There is in need for an improved electronic package comprising a gel TIM, which can reliability meet not only the end of line package performance requirements but also the end of life performance requirements.